Second what jim has said.
Lens optics are not necessarily even with the mount, believe it. Make the neg stage parallel to the base, then do the lens, but the final check is to check corners with grain magnifier. The Omega or Peak LONG Mirror model will see right into the corners.
I have name brand lenses that I have on separate boards because that is how they need to be shimmed to work best. I can not unscrew one and screw in a new one.
Changing neg carriers throws things off also as does changing print size if the column is not well braced.